Among various methods of saccharification of cellulose, enzymatic saccharification methods, which require small energy usage and provide high sugar yield, are considered as the mainstream of development. Cellulose is contained in a large amount in herbaceous plants and woody plants, and such plants are collectively referred to as cellulose-containing biomass. Cellulose-containing biomass contains hemicellulose such as xylan and arabinan and lignin in addition to cellulose. Xylan has a β-1,4-linked D-xylose backbone, and the backbone may be partially modified with O-acetyl, β-arabinofuranosyl, glucuronic acid, or a phenolic acid (Coughlan, M. P. et al., Biotechnol. Appl. Biochem. 17, 259-289 (1993)). Xylanase, which acts on the β-1,4-linked xylose backbone, is one of important enzymes to decompose cellulose-containing biomass.
Xylanases are classified into family 10 (GH10) and family 11 (GH11) on the basis of the homology of the amino acid sequence (Coutinho, P. M. et al., Recent Advances in Carbohydrate Bioengineering, 246, 3-12 (1999)). Xylanases belonging to GH10 generally have a molecular weight of 30 kDa or higher and, in contrast, xylanases belonging to GH11 are generally considered as having a relatively small molecular weight of approximately 20 kDa (Beaugrand et al., Carbohydr. Res. 339, 2529-2540 (2004)).
Filamentous fungi are known to be microorganisms to decompose a wide variety of cellulosic biomasses. Cellulase produced by Acremonium cellulolyticus in a culture solution is known to provide a higher glucose yield than cellulase produced by Trichoderma reesei in the decomposition of cellulose-containing biomass (Fujii et al., Biotechnol. Biofuels. 2, 24 (2009)). In recent years, seven types of xylanases have been cloned from Acremonium cellulolyticus, and the functions of the wild-type enzymes have been analyzed (Watanabe et al., AMB Express. 4, 27). Watanabe et al. report that XylC has the highest xylanolytic activity among the seven types of xylanases, although the expression level is the lowest in Acremonium cellulolyticus. 
It could therefore be helpful to provide a novel endoxylanase mutant having improved thermal stability. In addition, it could be helpful to provide an exogenous enzyme containing the same, and an efficient method of producing a sugar solution.
The Applicants hereby incorporate by reference the sequence listing contained in the ASCII text file titled SequenceListing.txt, created Apr. 19, 2017 and having 44.9 KB of data.